1. Field of the Invention
This invention relates to resonant converters such as commonly used as an electronic ballast for fluorescent and other gas discharge lamps, and more particularly to a resonant converter which can provide a change in operating condition, such as lamp turn-on or turn-off, while suppressing generation of high voltage and current transients.
2. Description of the Related Art
A well-known half-bridge resonant converter circuit which is commonly used as an electronic ballast for fluorescent and other gas discharge lamps is shown in FIG. 1A. Therein a power source 1 supplies a DC input voltage Vin across a pair of series connected electronic switches Q1, Q2 which may be power MOSFET""s, the gates of which are actuated by substantially square wave gating signals VGS1. and VGS2 which are complementary; i.e., VGS2 has a duty ratio D and VGS1 has a duty ratio (1xe2x88x92D), where D is the ratio of ON time to period of the signal. The power source 1 may comprise a full wave rectifier followed by a preconditioner circuit, as shown in U.S. pat. No. 5,742,134, issued Apr. 21, 1998, assigned to Philips Electronics N.A. Such gating signals are supplied by a conventional square wave generator 3. The frequency of the gating signals may be of the order of 45 kHz. Connected across switch Q2, in series, are a blocking capacitor Cb, inductor Lr and the primary winding of a step-up isolation transformer T having a primary magnetizing inductance Lm which is shunted by a capacitor Cr. The inductor Lr and capacitor Cr have a resonant frequency which is somewhat below the gating signal frequency. An output voltage V0 is produced across the secondary winding of the transformer T, and is applied to a fluorescent lamp connected thereto represented by a resistance R1 in series with a current limiting capacitance C1.
The circuit will be described starting from an initial state in which the lamp is off. It is assumed that the lamp is of the instant-start type which requires a starting voltage in the vicinity of 550 volts rms. During the xe2x80x9conxe2x80x9d portion or xe2x80x9c1xe2x80x9d state of each; a cycle of gating signal VGS1 it turns switch Q1 ON. The input voltage Vin, typically about 250 volts DC, then produces current in the path including Cb, Lr and Lm and also charges capacitance Cr. Capacitance Cb is much larger than Cr and serves only a blocking function to prevent DC from reaching transformer T. When gating signal VGS1 returns to the xe2x80x9c0xe2x80x9d state it turns switch Q1 OFF, and simultaneously gating signal VGS2 turns switch Q2 ON. The energy stored in inductance Lr and capacitance Cr then results in a very large reactive voltage across the transformer primary winding inductance Lm, since the voltage gain in the vicinity of the resonant frequency of Lr and Cr can be 10 to 15 or even greater. That is still further amplified by the step-up turns ratio of transformer T, and so it is readily achieved that adequate starting voltage is applied to the lamp to cause it to start-up.
After the lamp has ignited the lamp resistance R1 loads the resonant circuit, reducing the effective gain. Cyclic operation then continues at a frequency somewhat above the resonant frequency, producing a sufficiently high voltage to maintain the lamp in the ON condition. When it is desired to turn the lamp off the gating signal VGS1 applied to switch Q1 is turned off. It is thereby switched to the xe2x80x9c0xe2x80x9d state (duty ratio=0), remaining open, so that voltage is no longer supplied to the resonant circuit. At the same time, the common practice is to also turn gating signal VGS2 to the ON state (duty cycle=1), so that switch Q2 remains closed. The reactive energy existing in the resonant circuit at the instant turn-off is commenced will again result in a very large transient voltage across the transformer winding and also across inductance Lr which will then decay over an interval determined by the time constant of the reactive circuit loop.
The large reactive voltages and current produced in the converter circuit during lamp turn-on and turn-off requires the circuit designer to use circuit elements rated for voltages and currents many times the levels encountered during steady state circuit operation after the lamp has been turned on. That significantly increases the cost of the converter circuit. In addition, during turn-off the cyclic reactive voltage produced across the transformer winding may be sufficient to cause low frequency re-ignition of the lamp during several of the reactive cycles, causing repeated flickering which can be quite disturbing to an observer.
Accordingly, an object of the present invention is to provide a resonant converter which, while still providing an adequate instant-start voltage for a fluorescent or other gas discharge lamp, suppresses generation of the excessive transient reactive voltages and currents conventionally produced during lamp turn-on and turn-off. A further object is to provide definite turn-off of the lamp, without cyclic production of reactive voltages sufficient to cause low frequency repetitive re-ignition of the lamp.
Applicants have found that these objectives can be achieved by appropriate control of the duty ratios of the cyclic gating signals VGS1 and VGS2 which are supplied to the switches in the converter circuit. In particular, instead of immediate transition of each gating signal to or from the completely on or completely off state, the duty ratios of the gating signals are swept so as to incrementally reach the required altered duty ratio over several cycles of the gating signals. For lamp turn-on, the duty cycle of gating signal VGS1 is swept over several cycles from 0% (constant OFF) to 50% (ON time=OFF time), while simultaneously the duty ratio of gating signal VGS2 is swept over several cycles from 100% (constant ON) to 50% (ON time=OFF time). For lamp turn-off, the aforesaid sweeps of the duty cycles of the gating signals are reversed. Applicants have also found that control of the duty ratios of the gating signals may be employed to efficiently control the intensity of the light produced by the lamp, as contrasted with conventional control of light intensity by alteration of the cyclic frequency of the gating signals.
Such gradual alteration of the duty ratios of the cyclic gating signals has been found to substantially suppress generation of high voltage and current transients during lamp turn-on or turn-off, as well as low frequency repeated re-ignition of the lamp during turn-off.